Flexible circuit board interconnect with strain relief

ABSTRACT

A flexible circuit board interconnect is provided with flex strain relief capabilities. The flexible circuit board interconnect includes a polyamide or polyimide (both are nylons) substrate layer, a ground plane layer and a conductor strip layer. The ground plane layer is provided with scalloped edges along opposite sides thereof. The ground plane provides a shield for preventing radio frequency interference to and from the signals passed through the conductor strips. The ground plane layer and the conductor strips are covered at least partially by a pair of soldermask layers which are provided with scalloped edges. The scalloped edges on the soldermask layers and the ground plane prevent the generation of a crease or fold line on the surface of the flexible circuit board interconnect. Each of the conductor strips are provided with a pair of vias on opposite ends of thereof. The vias are arranged to be offset from the vias on adjacent conductor strips such that a perforation-type tear line is avoided and such that when a hot bar soldering is used, at least one via remains uncovered for receiving soldering wicking, thus reducing the likelihood of solder bridging across conductor strips.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to a circuit board interconnect, and moreparticularly, to an improved flexible circuit board interconnect withflex strain relief.

BACKGROUND AND SUMMARY OF THE INVENTION

In the automotive and aircraft industries, on-board computer systems andcomputer control systems have become commonplace. With more and more ofthe vehicle systems becoming computer-controlled and operated, the needfor connecting multiple circuit boards on a vehicle or aircraft hasgreatly increased over the last decade. Currently, vehicles are beingprovided with circuit boards which require dozens of electricalconnections to be made to other circuit boards on the vehicle.Accordingly, several attempts have been made at providing electricalinterconnects for connecting the vehicle circuit boards. However, theseinterconnects suffer from several disadvantages.

A first disadvantage of conventional interconnects is that they aresusceptible to electromagnetic interference and also can generateelectromagnetic interference. This electromagnetic interference cancause malfunctions in vehicle systems.

Furthermore, the conventional circuit board interconnects are ofteneasily damaged. Repeated bending of the conventional flexibleinterconnects often leads to cracking or tearing.

The conventional interconnects are also difficult to assemble to thecircuit boards. If the flexible circuit board interconnects are tooflexible, there is often a problem with damaging the conductor stripswhile aligning the flexible interconnects.

Also, circuit board interconnects must provide sufficient groundingcapabilities. The ground requirements for typical circuit boards can beseveral amperes.

Accordingly, it is an object of the present invention to provide acircuit board interconnect with flex/strain relief capabilities.

It is a further object of the present invention to provide a circuitboard interconnect with sufficient ground capabilities.

It is still another object of the present invention to provide a circuitboard interconnect with sufficient heat resistance and thermal processmanagement capability.

It is still another object of the present invention to provide a circuitboard interconnect with a ground shield for preventing radio-frequencyinterference to and from the signals passed through the circuit boardinterconnect.

These and other objects of the present invention are obtained byproviding a flexible circuit board interconnect, including a flexible,generally flat, substrate having first and second opposing faces. Aground plane including a plurality of ground strips is disposed on thefirst face of the substrate. A plurality of parallel conductor stripsare disposed on the second side of the substrate. A first soldermask isdisposed on the first face of the substrate over top of the groundshield. A second soldermask is disposed on the second side of thesubstrate over top of the conductor strips. The first and secondsoldermask layers are provided with scalloped edges.

According to another aspect of the present invention a flexible circuitboard interconnect is provided with a pair of vias through each of thecopper pad strips and opposite ends of each of the conductor strips. Thevias are disposed in a staggered arrangement on each of the copper padstrips.

According to still another aspect of the present invention, a flexiblecircuit board interconnect is provided with a ground shield made ofcopper disposed on the first surface of the substrate. The ground shieldincludes a plurality of ground strips extending from one side to asecond side of the substrate.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood however that the detailed description and specificexamples, while indicating preferred embodiments of the invention, areintended for purposes of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a perspective view of the circuit board interconnect of thepresent invention connecting contact pads on a first circuit board tocorresponding contact pads on a second circuit board;

FIG. 2 is a plan view of the top surface of the circuit boardinterconnect according to the present invention;

FIG. 3 is a plan view of the bottom surface of the circuit boardinterconnect according to the present invention;

FIG. 4 is an enlarged detail view of the circled area 4 of FIG. 3,illustrating the alternating pattern of the vias provided in the ends ofthe conductor strips;

FIG. 5 is a cross-sectional view taken along line 5--5 of FIG. 2;

FIG. 6 is an enlarged plan view illustrating the placement of a solderbar over the centrally located vias of the conductor strips duringassembly;

FIG. 7 is a plan view of the bottom soldermask layer according to thepresent invention;

FIG. 8 is a plan view of the ground plane layer according to the presentinvention;

FIG. 9 is a plan view of the polyimide substrate layer according to thepresent invention;

FIG. 10 is a plan view of the conductor strip layer according to thepresent invention; and

FIG. 11 is a plan view of the top soldermask layer according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1-11, the flexible circuit board interconnect 10according to the present invention will be described. FIG. 1 illustratesthe flexible circuit board interconnect 10 connected to a first set ofcontact pads 12 of a first circuit board 14, and a second set of contactpads 16 of a second circuit board 18.

With reference to FIGS. 2-11, the flexible circuit board interconnect 10according to the present invention will now be described in detail. Asbest seen in FIG. 5, flexible circuit board interconnect 10 includes asubstrate layer 20 and a ground plane layer 22 disposed on a firstsurface 24 of substrate 20. A conductor strip layer 26 is provided on asecond surface 28 of substrate 20. A top soldermask layer 30 is disposedon ground plane layer 22. A bottom soldermask layer 32 is disposed onconductor strip layer 26. Substrate 20 is preferably made from a nylonmaterial, such as polyimide, for example NOVACLAD™, available fromSheldahl Corp., Northfield, Minn. 55057. However, it should berecognized that other materials having sufficient flexibility,durability, and heat resistance may be used. The top and bottomsoldermask layers are formed from known soldermask materials.Preferably, the soldermask layers are made from an acrylic photoimageable curable soldermask, commercially available from DuPont Corp.

As seen in FIGS. 3 and 10, the bottom surface of the flexible circuitboard connector 10 includes the conductor strip layer 26 which includesa plurality of parallel conductor strips 34 which extend substantiallyfrom a first edge 36 of the polyimide substrate layer 20 to a secondedge 38 of the polyimide substrate layer 20. Each of the conductorstrips 34 include first and second ends 34a, 34b which are provided withtwo vias or holes 40 passing therethrough, as shown in FIG. 4. The vias40 are disposed in an alternating offset or staggered fashion such thatthe vias 40 on one conductor strip 34 are not disposed next to the vias40 on an adjacent conductor strip. Furthermore, at least one via 40 oneach connector strip 34 remains uncovered when a hot bar solderingprocess is used. As shown in FIG. 6, a hot solder bar 42 is laid acrossthe end portions 34a, 34b of the conductor strips in order to solder thecircuit board interconnect 10 to contact pads 12 and 16 of first andsecond circuit boards 14, 18, respectively. The vias 40, which are notdisposed under the solder bar 42, are available for soldering wicking,thus reducing the likelihood of solder bridging across conductor strips34. In addition, because vias 40 are not aligned with one another onadjacent conductor strips 34, a perforation effect is avoided so that abreaking or folding location is not created when the flexible circuitboard connector 10 is bent. Vias 40 also provide heat conduction throughthe substrate to lower the thermal resistance during a hot bar solderingprocess. Along these lines, it is sufficient if the inboardmost vias areoffset from one another irrespective of the alignment of the other vias.In this way, the perforation effect is avoided.

A bottom soldermask layer 32 is disposed over top of the centralportions of each conductor strip 34. As best seen in FIG. 7, bottomsoldermask layer 32 is provided with a scalloped edge 44 correspondingwith the first and second edges 36, 38 of the flexible circuit boardinterconnect 10.

With reference to FIGS. 2 and 8, the top surface of the circuit boardinterconnect 10 will be described. The top surface of the flexiblecircuit board interconnect 10 includes a ground plane layer 22 whichincludes a ground shield 48. Ground shield 48 preferably includes crosshatched copper strips forming a mesh-like configuration. According tothe invention, the ground shield provides a low-impedance ground pathacross the interconnect 10, and also provides radio frequencyinterference shielding. The outer edges of the ground shield 48 areprovided with a copper strip forming scalloped edges 54 which extendalong the first and second edges 36, 38 of the flexible circuit boardinterconnect 10. A plurality of ground strips 56 extend from the firstedge 36 to the second edge 38 of the top surface of the flexible circuitboard interconnect 10. The ground strips 56 are disposed directly abovecorresponding interconnect strips 34 disposed on the bottom surface offlexible circuit board interconnect 10. A plurality of copper pads 58are also disposed on the top surface of the flexible circuit boardinterconnect 10. The copper pads 58 are disposed opposite acorresponding conductor strip 34 disposed on the bottom surface of theflexible circuit board interconnect 10. The copper pads 58 extend fromone of the first and second edges 36, 38 of the flexible circuit boardinterconnect 10 toward one of the scalloped edges 54 of ground shield 48such that the copper pads 58 do not contact the ground shield 48.

A top soldermask layer 30 is disposed over top of ground shield 48 andpartially covers the ends of a plurality of copper pad strips 58. Theedges 60 of top soldermask layer 30 are scalloped, as best shown in FIG.11. In other words, the edges 60 include a series of rounded peaksdisposed on opposite sides of a series of rounded valleys. Preferably,the peaks of the scalloped edges 60 of the top soldermask layer 30 andthe peaks of the scalloped edges 44 of the bottom soldermask layer 32are 180° out of phase with one another, as best seen in FIG. 4. Thescalloped edges 44 and 60 provide strain relief and eliminate the needfor a separate strain relief strip typically required for interconnectsof this type. The 180° phase shift reduces the likelihood of tearingalong the edges 36, 38 of the circuit board interconnect 10.

The flexible circuit board interconnect 10 is provided with four guideholes 66a-66d disposed at opposite corners thereof. The guide holes66a-66d are provided for aligning the flexible circuit boardinterconnect 10 relative to first and second circuit boards 14, 18.Circuit boards 14, 18 can be provided with guide pins (not shown) whichare received through guide holes 66a-66b of the flexible circuit boardinterconnect 10. The guide holes 66a-66d can be strategically locatedsuch that the flexible circuit board interconnect 10 can only be engagedwith the locator pins on the first and second circuit boards 14, 18 ifthe flexible circuit board interconnect 10 is in the proper orientation.Furthermore, one or more of the guide holes can be elongated toaccommodate heat expansion and other manufacturing tolerances. Aplurality of reinforcement rings 68a-68b are optionally provided aroundguide holes 66a-66b for providing additional protection against tearingof the substrate layer 20 in the area of the guide holes 66a-66b.Reinforcement rings 68a-68d can be made of copper, plastic, or anothersuitable material. Copper is preferably used since it is already beingapplied with the ground shield 48 and connector strips 34. It should beunderstood that the reinforcement rings 68a-68b can be utilized aroundeach of the guide holes 66a-66d.

As discussed above, the copper ground shield 48 is provided with aplurality of ground strips 56 that connect to grounding pads on each ofthe circuit boards when the interconnect 10 is attached to the printedcircuit boards 14, 18. The ground shield 48 can carry a certain amountof amperage per ground strip and combine with the ground shield 48 toform a ground plane. The ground shield 48 enhances electromagneticinterference reduction. Electromagnetic interference with the conductorstrips 34 can cause interference with the signals passed throughconductor strips 34. Ground shield 48 can be formed of a solid layer ofelectrically conductive material such as copper or from a mesh-likeconfiguration of conductive material for added flexibility and weightreduction.

It should also be noted that the principles of the present invention canalso be applied to a shielding mechanism utilized solely for shielding acircuit board from electromagnetic interference. According to thisaspect of the invention, a shield made from conductive material isprovided on a surface of a flexible substrate. The flexible substrate ismounted in a covering relationship over top of a circuit board in orderto shield the circuit board from electromagnetic interference.Preferably, the shield is electrically grounded so that anyelectromagnetic energy absorbed by the shield can be dissipated.

The scalloped edges 60, 44, and 54 provided on the soldermask layers 30,32 and the ground shield 48 reduce the likelihood of tearing along theedges 36,38 of the flexible circuit board interconnect 10. For example,if the ground shield or the soldermask layers were provided withstraight edges, a crease or a fold line would be built into the flexiblecircuit board interconnect 10. However, with the scalloped surfaceformed along the edges of the ground shield 48 and the soldermask layers30, 32, the creation of a crease or fold line is avoided.

The alternating position of the vias 40 in the ends of the conductorstrips 34 also avoid the effect of generating a perforation-type lineacross the flexible circuit board interconnect 10. In addition, bypositioning the vias 40 in the ends of the conductor strips so that atleast one via 40 remains uncovered in each conductor strip 34 when a hotbar soldering process is used, a hole is available for solderingwicking, thus reducing the likelihood of solder bridging acrossconductor strips 34.

According to a preferred embodiment, the polyimide substrate layer 20 isprovided with a copper layer on first and second opposite sides thereof.The copper layers are etched away, using known processes, such that thecopper conductor strips 32, the ground shield 48, and the copper padstrips 56 remain. The vias 40 are preferably drilled through theconductor strips 34, the polyimide substrate 20 and the copper padstrips 56. The soldermask layers 30, 32 can be applied before or afterthe vias 40 are drilled.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A flexible circuit board connector, comprising:aflexible flat substrate having first and second surfaces; a plurality ofparallel conductor strips disposed on said second surface of saidflexible flat substrate; and a current carrying ground shield disposedon said first surface of said flexible flat substrate, said groundshield having first and second substantially rounded scalloped edges;and a first soldermask layer disposed over said ground shield.